UPDATE!
Further to ancestry.co.uk working their way through the London marriage registers - I have found another marriage for Elizabeth Mountain - to Edward Ware on 7 October 1886. Elizabeth, again gives her father as "William Mountain, Station Master", and the signatures match.
Following this, the mysterious "Mr Pearce" is identified (Elizabeth married her last husband as "Elizabeth Pearce, Widow"). Elizabeth Ware, Widow, married Kenneth Hamilton Pearce on 24 December 1897. Again, her father is "William Mountain, Station Master (retired)". Unfortunately, this one is not on ancestry, yet, so I've had to buy the certificate and of course it is not her original signature to compare.
Edward Ware started divorce proceedings in July 1890 against Elizabeth and John Le Bos (co-respondent). I don't think a divorce was granted, however - I can't find any reference to it in the newspapers. Six months later Edward was prosecuted for taking a friend round to Elizabeth's house and assaulting her. The only John Le Bos I can find is someone from Jersey?
Kenneth Pearce was admitted to a private asylum in 1901, and later emigrated to Australia, where he died in 1920.
I think Edward Ware (born 1860 in Bermondsey), later lived with a woman called Florence Golby in Sheffield. I cannot find him after the 1911 Census, when he is at 23 Sambourne (or Lambourne) Square, Sheffield.
I am now investigating a possible 6th marriage - to James Duke, registered in the June quarter 1886 at Pancras - the same year she married Edward Ware.